Steering functionality in an automotive vehicle is typically enabled via a series of mechanical linkages between a steering input device, e.g., a steering wheel, and the front road wheels of the vehicle. For example, in a conventional rack-and-pinion device, rotation of the steering wheel rotates a pinion gear within a pinion housing. The pinion gear is continuously engaged along a series of notches or teeth of an elongated rack portion of the rack-and-pinion device. Tie rods connect the rack to the front road wheels, such that any rotation of the steering wheel ultimately forces or moves the rack left or right as needed to achieve the desired orientation of the road wheels during a steering maneuver. The rack-and-pinion device can be configured to produce a desired steering ratio to optimize steering performance, depending largely on the number and design of the pinion gears used therein.
In an electric power steering (EPS) system in particular, such as an exemplary dual rack-and-pinion style EPS system as described herein, movement of the rack portion of the rack-and-pinion device described above is electrically assisted via a controlled application of a variable motor torque from an electric steering motor, with an applied torque for steering of the vehicle also selectively adjusted via a torque overlay command or TOC as determined by onboard control logic. That is, the level of steering “assist” is determined via an EPS electronic control unit or ECU. Within an EPS system, vehicle speed and steering wheel dynamics such as steering angle and steering torque are continuously monitored to determine how much steering assist is required for a particular steering maneuver. Once the ECU has determined an appropriate amount of assist to apply in a given scenario, the output of the steering motor is varied to produce a corresponding amount and direction of rotation, thus modifying the motion of the rack. The level of assist can vary depending upon changes in vehicle speed and other dynamic inputs, such as but not limited to signals or inputs from an electronic stability control and/or an electronic traction control system aboard the vehicle.
A state of the art EPS system can provide multiple operating modes or states depending on the required application and/or the level of assist. During normal steering during which a driver applies a positive torque to the steering wheel, the EPS system can assist the right/left steering direction. Additionally, an EPS system can assist the rate of return of the steering wheel to a center or neutral position upon completion of a turning maneuver, or can help maintain a lane position of the vehicle within minimal or no steering input from the driver by means of the torque overlay command (TOC). Finally, some EPS systems have additional modes, e.g., damping and/or overload or protection modes, that each provides an additional safeguard against overload or overheating of the steering motor, and/or provide an optimized stability response.
During any of these exemplary EPS modes or states, the delivery or transmission of at least some level of assist from the ECU can be expected. In an active torque overlay operation, i.e., when a torque overlay command (TOC) is actively occurring in addition to a calculated amount of motor torque from the steering motor, it is expected that the steering torque from a driver to the steering wheel is kept at a relative minimum, down to and possibly including a zero value. In other words, a driver may simply grip the wheel lightly and passively follow any autonomous rotation of the steering wheel under control of the EPS system without actively applying a steering torque to the steering wheel, or by applying only a minimal amount of steering torque. During certain collision avoidance and/or stability steering maneuvers, however, the driver may wish to rapidly establish full authority or control over the steering maneuver.